1. Field of the Invention
The present invention relates to methods and apparatus for using optical fibers as communication transmission paths, and more specifically to the combining of two separate RF signals such as CATV signals and Direct Broadcast Satellite (DBS) signals on a single optical fiber. The invention is also related to at least partially substantially controlling the apparatus.
2. Description of Related Art Including Information Disclosed Under 37 CFR 1.97 And 1.98
The communication industry is using more and more optical or light fibers in lieu of copper wires since optical fibers have substantially greater bandwidth than traditional copper wires or cables. For example, in addition to bidirectional telephone signals, systems are now coming on-line which use a single optical fiber to transmit cable TV, (Community Antenna TV commonly referred to as CATV) signals along with the bidirectional telephone signals. In a typical installation, the bidirectional telephone signals will be transmitted as Time Compressed Multiplexed (TCM) signals which modulate light having a nominal light wavelength of about 1310 nanometers, and the unidirectional TV or RF signals are used to modulate light having a wavelength of about 1550 nanometers. The two wavelengths of light are then combined by an optical coupler such as by Wave Dimension Multiplexing (WDM) onto a single optical fiber. This approach works well, but there are ever increasing demands for other types of communication techniques. For example, Direct Broadcast Satellite (DBS) is now available and provides a digital TV signal which is typically coded and compressed and consequently is much less subject to degradation and interference by noise signals. It is now desirable to provide DBS signals to the home other than by an individual satellite dish. To date, there is no effective and acceptable commercial method of such signal distribution other than by dedicated cables.
Unfortunately, some of the equipment necessary for such systems can be extremely expensive, complex and sensitive. Consequently, the task of continuously controlling and monitoring such equipment is very important if the equipment is to be dependable.
Therefore, it is an object of the present invention to provide acceptable and effective commercial methods and apparatus for combining two separate RF signals onto a single optical fiber.
It is another object of the present invention to provide a direct RF signal to a modulated light converting system which is easy to troubleshoot and repair by providing indications and alarms in the event of failure or poor performance.
These and other objects are achieved by the apparatus of this invention which comprises a first optical fiber for providing light at a selected nominal wavelength. The provided light is modulated by an RF signal such as for example a cable TV signal having a frequency in the range of 50 MHz to 870 MHz. The nominal wavelength of light used for carrying the RF signal is typically on the order of 1556 nanometers. A direct RF signal such as a Digital Broadcast Satellite (DBS) signal which is typically received by a satellite dish is also made available and will typically have a frequency band of between 950 and 2050 MHz. It is seen that there is at least 80 MHz separation between the two RF signals. (950-870). The RF signal is then passed through a high pass filter which is designed to readily pass signals having a frequency within the frequency band of the RF signal and which eliminates or substantially reduces signals which are in the frequency band of the previously discussed CATV RF signal. The digital DBS signal leaving the high pass filter is then provided to an optical light generator which generates and directs light onto a second optical fiber at a second wavelength, having a nominal value of 1548 nanometers. The filtered digital DBS signal is used to modulate this generated light. Although it may be possible to use various types of optical light generators, a Distributed Feedback Bragg (DFB) laser is particularly suitable for the present invention because of its very narrow bandwidth. The two optical fibers carrying the CATV RF signal and the DBS RF signal are then provided to an optical coupler which is selected to provide a combined output on a third optical fiber. The optical coupler is selected such that more than 50% of the combined light output is provided by the first optical fiber and should have a first selected percentage such as, for example, between about 55% and 80% and preferably about 70%. The first optical fiber carries the CATV RF signal. The second optical fiber which is carrying the DBS RF signal provides a second selected percentage of between about 45% and 20% of the combined light and preferably about 30%. These experimentally determined percentages have been found to be of significant importance because of the high susceptibility of the CATV RF signals to noise and spurious other signals. In a preferred arrangement, the DBS RF signal will be approximately 6 dB less than the CATV RF signal and the two signals will be separated by about 6-10 nanometers.
The system further includes circuitry for monitoring the optical signal and the DBS RF signal and sending alarms when these signals deviate from preset limits.